shared.unattended: Use fixed partition for RHEL5/6 guests

Currently we use autopart in all rhel guests, which bring
trouble when RHEL5 RHEL6 guests w/ huge(64+G) RAM. The
swap partition will be really huge and not enough space
for the root partition, thus installation fails.
(we met this problem on huge RAM host before).

With this patch, the RHEL5 RHEL6 guest will have only 2
physical partitions(/boot, PV), and the PV contains 2
 LVs(/, swap).

|     | /    |swap|
|/boot|----LVM----|
|     |    PV     |
|---------------- |
|guest hard drive |

Note:
1) we can use plain partitions here (w/o LVM) to gain the
   possible tiny performance improvement, but I prefer to
   choose the LVM scheme, a little bit complex partioning
   scheme but test more (also testing the lvm layer).
2) RHEL3, RHEL4, RHEL7 kickstart config didn't update here,
   because:
   The default kernel shipped w/ RHEL3, RHEL4 dont support
   such huge RAM, so they will not met this issue.
   The anaconda of RHEL7 looks fixed this problem, install
   w/ autopart will not create a huge swap partition.
Signed-off-by: NXiaoqing Wei <xwei@redhat.com>
Acked-by: NSuqin Huang <shuang@redhat.com>
Signed-off-by: NQingtang Zhou <qzhou@redhat.com>